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IEC 61972: Method for Determining Losses in Induction Motors — Precision Measurement and Efficiency Classification
✅ Standard at a Glance
IEC 61972, published in 2002 by IEC Technical Committee 2 (Rotating machinery), establishes a unified method for determining losses and efficiency of three-phase induction motors. It harmonizes earlier national standards (IEEE 112-B, IEC 60034-2) into a single global procedure that specifies test methods, loss segregation techniques, and correction factors. The standard applies to low-voltage and medium-voltage induction motors up to 13.8 kV and covers a power range from fractional kilowatts to several megawatts.
IEC 61972, published in 2002 by IEC Technical Committee 2 (Rotating machinery), establishes a unified method for determining losses and efficiency of three-phase induction motors. It harmonizes earlier national standards (IEEE 112-B, IEC 60034-2) into a single global procedure that specifies test methods, loss segregation techniques, and correction factors. The standard applies to low-voltage and medium-voltage induction motors up to 13.8 kV and covers a power range from fractional kilowatts to several megawatts.
🔌 1. Loss Segregation Methodology
1.1 The Five Categories of Motor Losses
IEC 61972 mandates that induction motor losses be segregated into five distinct categories, each with its own measurement or calculation method:
| Loss Category | Symbol | Measurement Method | Key Influencing Factors | Typical % of Total Losses (4-pole, 75 kW) |
|---|---|---|---|---|
| Stator I²R losses (copper) | Ps | DC resistance measurement corrected to operating temperature | Winding temperature, conductor cross-section, fill factor | 30-40% |
| Rotor I²R losses (copper/aluminium) | Pr | Calculated from slip measurement and air-gap power | Rotor bar material, cage design, operating slip | 20-30% |
| Core losses (iron) | Pfe | No-load test at rated voltage and frequency | Lamination grade, flux density, manufacturing stress | 15-25% |
| Friction and windage losses | Pfw | No-load test extrapolated to zero voltage | Bearing type, lubrication, fan design, speed | 5-15% |
| Stray load losses | Psl | Residual method or assigned value (0.5% of input power) | Slot harmonics, skin effect, leakage flux | 5-15% |
The standard requires that the total loss be calculated by summing these five components, from which the efficiency is obtained as:
η = Pout / (Pout + ΣLosses) × 100%
Where Pout is the mechanical output power measured by a dynamometer or torque transducer, and ΣLosses is the sum of all five segregated loss components.
💡 Engineering Insight
The stray load loss category is the most controversial and difficult to measure precisely. IEC 61972 offers two approaches: the residual method (preferred), where stray loss is calculated as the remaining unaccounted loss after all other losses are subtracted from the total measured loss, and the assigned value method, which uses a fixed value of 0.5% of the rated input power. In practice, the residual method gives more accurate results for larger machines, while the assigned method is a conservative approximation suitable for smaller motors. Experienced test engineers know that the accuracy of stray loss determination directly impacts the IE classification outcome.
The stray load loss category is the most controversial and difficult to measure precisely. IEC 61972 offers two approaches: the residual method (preferred), where stray loss is calculated as the remaining unaccounted loss after all other losses are subtracted from the total measured loss, and the assigned value method, which uses a fixed value of 0.5% of the rated input power. In practice, the residual method gives more accurate results for larger machines, while the assigned method is a conservative approximation suitable for smaller motors. Experienced test engineers know that the accuracy of stray loss determination directly impacts the IE classification outcome.
1.2 Temperature Correction and Reference Conditions
A critical aspect of IEC 61972 is the requirement to correct all loss measurements to reference operating temperatures. Stator winding resistance must be corrected from the test temperature to the reference temperature using the standard temperature coefficient of copper (0.004 K-1 at 20 °C). The reference temperature depends on the insulation class:
| Insulation Class | Reference Temperature | Winding Temperature Rise Limit |
|---|---|---|
| A | 80 °C | 60 K |
| E | 95 °C | 75 K |
| B | 100 °C | 80 K |
| F | 120 °C | 105 K |
| H | 145 °C | 125 K |
Without proper temperature correction, the reported efficiency can vary by 0.5-1.5 percentage points, which is sufficient to shift a motor from one IE class to another.
💡 2. Test Procedures and Instrumentation Requirements
2.1 No-Load and Locked-Rotor Tests
IEC 61972 specifies two fundamental tests that form the basis of loss segregation:
No-load test: The motor is run at rated voltage and frequency without any mechanical load. The input power measured under this condition represents the sum of core losses and friction/windage losses. By performing a series of no-load measurements at varying voltages (typically from 125% down to 20% of rated voltage) and plotting input power versus voltage squared, the core loss and friction/windage components can be separated through linear regression extrapolation to zero voltage.
Locked-rotor test: The rotor is mechanically locked, and a reduced voltage (typically 15-30% of rated voltage) is applied to circulate rated current. This test determines the combined stator and rotor I²R losses at standstill and provides data for calculating the equivalent circuit parameters needed for slip-dependent loss calculations.
⚠️ Design Warning
A frequently overlooked requirement in IEC 61972 is the stabilization period before no-load testing. The standard mandates that the motor must run at no-load for a sufficient time (typically 30-60 minutes for small motors, several hours for large machines) to reach thermal equilibrium. Premature measurement can result in core loss errors of 10-15% because the winding resistance has not yet stabilized. Engineers conducting certification tests should monitor winding temperature continuously and only begin data collection when the temperature change is less than 1 K in 30 minutes.
A frequently overlooked requirement in IEC 61972 is the stabilization period before no-load testing. The standard mandates that the motor must run at no-load for a sufficient time (typically 30-60 minutes for small motors, several hours for large machines) to reach thermal equilibrium. Premature measurement can result in core loss errors of 10-15% because the winding resistance has not yet stabilized. Engineers conducting certification tests should monitor winding temperature continuously and only begin data collection when the temperature change is less than 1 K in 30 minutes.
2.2 Instrumentation Accuracy Classes
IEC 61972 specifies minimum accuracy requirements for test instrumentation, which vary depending on the target uncertainty:
| Measurement | Routine Test (Class 2) | Precision Test (Class 1) | Reference Test (Class 0.5) |
|---|---|---|---|
| Voltage measurement | ±0.5% | ±0.2% | ±0.1% |
| Current measurement | ±0.5% | ±0.2% | ±0.1% |
| Power measurement | ±1.0% | ±0.5% | ±0.2% |
| Torque measurement | ±1.0% | ±0.5% | ±0.2% |
| Speed measurement | ±1 r/min | ±0.5 r/min | ±0.1 r/min |
| Resistance measurement | ±0.5% | ±0.2% | ±0.1% |
The choice of instrumentation class has a direct impact on the combined uncertainty of the efficiency determination. For IE4 (Super Premium Efficiency) motors, where the efficiency can exceed 96%, even a Class 1 test setup may yield an uncertainty of ±0.4 percentage points, which represents a significant fraction of the efficiency band.
💻 3. Engineering Design Insights and Practical Applications
3.1 Stray Load Loss Reduction Techniques
The determination and minimization of stray load losses is where IEC 61972 provides the most valuable engineering guidance. Stray losses originate from harmonic flux components in the air gap, induced eddy currents in winding conductors, and leakage flux in the end-winding region. Modern design techniques that reduce stray losses include:
Optimized slot geometry: Using semi-closed or closed rotor slots reduces the amplitude of slot-opening harmonics that induce high-frequency losses in the stator and rotor surfaces. The slot opening width should be minimized to less than 3% of the slot pitch for optimal stray loss performance.
Magnetic slot wedges: Replacing non-magnetic slot wedges with magnetic wedges (typically ferrite-polymer composites with relative permeability of 3-10) reduces the effective slot opening and suppresses permeance harmonics. This can reduce stray load losses by 15-25% in medium-voltage motors.
Rotor bar insulation: In large fabricated rotor designs, insulating the rotor bars from the core laminations prevents inter-bar currents that contribute to stray losses. This technique is essential for motors above 500 kW where stray losses would otherwise increase disproportionately with machine size.
✅ Stray Loss Reduction Case Study
A 1.5 MW, 6-pole medium-voltage induction motor originally designed with open stator slots and uninsulated rotor bars showed stray load losses of 12.5 kW (0.83% of rated power). After redesigning with magnetic slot wedges and insulated rotor bars, the stray losses dropped to 7.2 kW (0.48% of rated power), improving the overall efficiency from 95.8% to 96.3% and moving the machine from IE3 to IE4 classification. The additional manufacturing cost was recovered within 18 months of operation through energy savings.
A 1.5 MW, 6-pole medium-voltage induction motor originally designed with open stator slots and uninsulated rotor bars showed stray load losses of 12.5 kW (0.83% of rated power). After redesigning with magnetic slot wedges and insulated rotor bars, the stray losses dropped to 7.2 kW (0.48% of rated power), improving the overall efficiency from 95.8% to 96.3% and moving the machine from IE3 to IE4 classification. The additional manufacturing cost was recovered within 18 months of operation through energy savings.
3.2 Practical Considerations for Efficiency Testing
Engineers performing efficiency tests per IEC 61972 should be aware of several practical factors that significantly influence results:
Supply voltage quality: The standard assumes a sinusoidal, balanced supply. Voltage unbalance of just 1% can increase motor losses by 2-5% due to negative-sequence currents. Tests should only be conducted when the voltage unbalance factor (VUF) is below 0.5%, and results should be corrected to ideal conditions using the symmetrical component method described in Annex B of the standard.
Torque transducer alignment: Misalignment between the test motor and the dynamometer or load machine introduces parasitic torque errors. The standard recommends a shaft alignment tolerance of less than 0.05 mm parallel offset and 0.05° angular misalignment. Even small misalignments can cause torque measurement errors of 0.2-0.5%, which directly translate to efficiency errors of similar magnitude.
Ambient temperature effects: Testing at ambient temperatures significantly different from the reference temperature (25 °C ± 5 K is recommended) requires careful application of temperature correction factors. For every 10 K deviation from the reference, the winding resistance correction introduces an approximately 4% change in I²R losses.
⛾ 4. Relationship with the IE Efficiency Classification System
IEC 61972 provides the measurement foundation for the IE efficiency classification system defined in IEC 60034-30-1. The standard defines the testing protocols that determine which IE class (IE1, IE2, IE3, IE4) a motor achieves. The relationship between the test method and the classification system is critical:
| IE Class | Efficiency Level | Typical Efficiency (75 kW, 4-pole) | Required Test Method | Year Introduced |
|---|---|---|---|---|
| IE1 | Standard Efficiency | 93.0% | IEC 61972 or IEC 60034-2-1 | 2008 |
| IE2 | High Efficiency | 94.3% | IEC 61972 (summation of losses) | 2008 |
| IE3 | Premium Efficiency | 95.4% | IEC 61972 (precision, with stray loss measurement) | 2008 |
| IE4 | Super Premium Efficiency | 96.2% | IEC 61972 (reference test, Class 0.5 instruments) | 2014 |
As of 2026, many jurisdictions worldwide have mandated IE3 as the minimum efficiency level for general-purpose motors, with IE4 becoming mandatory for motors above 75 kW in the European Union. The precision of the IEC 61972 test method thus has direct regulatory and commercial implications.
🔴 Critical Engineering Note
When using IEC 61972 for warranty verification or regulatory compliance, the standard explicitly requires that the uncertainty of measurement be reported alongside the efficiency value. A common point of dispute in motor procurement contracts arises when the manufacturer’s test (typically Class 2) reports an IE3 efficiency of 95.5%, but the user’s acceptance test (Class 1) measures 95.0%. The difference may fall within the combined uncertainty of both measurements. IEC 61972 provides guidance on calculating the expanded uncertainty (k=2, 95% confidence level) to resolve such disputes. Always specify the test method and accuracy class in procurement specifications.
When using IEC 61972 for warranty verification or regulatory compliance, the standard explicitly requires that the uncertainty of measurement be reported alongside the efficiency value. A common point of dispute in motor procurement contracts arises when the manufacturer’s test (typically Class 2) reports an IE3 efficiency of 95.5%, but the user’s acceptance test (Class 1) measures 95.0%. The difference may fall within the combined uncertainty of both measurements. IEC 61972 provides guidance on calculating the expanded uncertainty (k=2, 95% confidence level) to resolve such disputes. Always specify the test method and accuracy class in procurement specifications.
❓ Frequently Asked Questions
❔ What is the difference between IEC 61972 and IEC 60034-2-1?
IEC 61972 (2002) was the first international standard dedicated to induction motor loss determination. It was later superseded and harmonized into IEC 60034-2-1 (2007, amended 2014), which now serves as the primary reference. However, IEC 61972 remains referenced in many existing product specifications and regulatory documents. The fundamental methodology is the same — segregated losses, summation of losses method, temperature correction — but IEC 60034-2-1 provides updated guidance for converter-fed motors and extends the scope to synchronous machines.
❔ How does the standard handle motors driven by variable frequency drives (VFDs)?
The original IEC 61972 was developed primarily for sinusoidal supply testing. For VFD-fed motors, additional losses from inverter-induced harmonics (high-frequency switching losses, additional core losses from time harmonics) must be considered. IEC 60034-2-1 (and its amendment) introduced specific methods for determining losses under converter supply, including sinusoidal-fed testing with additional loss allowances and direct testing with a defined converter supply. Engineers should consult IEC 60034-2-1 Annex C for detailed guidance on converter-fed motor efficiency testing.
❔ Can IEC 61972 be applied to single-phase induction motors?
No. IEC 61972 is specifically limited to three-phase induction motors. Single-phase induction motors, which operate with an auxiliary winding and typically a starting capacitor, have fundamentally different loss distributions (higher copper losses in the auxiliary winding, additional capacitor losses) that are not addressed by this standard. For single-phase motors, engineers should refer to IEC 60034-2-1 (which includes some single-phase provisions) or national standards such as NEMA MG-1.
❔ What is the typical uncertainty of efficiency determination using IEC 61972?
The combined uncertainty depends strongly on the instrumentation class and test procedure. For a Class 1 precision test on a 100 kW motor, the expanded uncertainty (k=2) is typically ±0.3 to ±0.5 percentage points. For a Class 2 routine test, it increases to ±0.5 to ±1.0 percentage points. For very high-efficiency machines (>96%), the uncertainty can be a significant fraction of the measured losses, making it difficult to distinguish between IE3 and IE4 with high confidence. The most significant uncertainty contributors are torque measurement (typically 40% of total uncertainty), power measurement (30%), and stray loss determination (20%).
